Radio communication networks were originally developed primarily to provide voice services over circuit-switched networks. The introduction of packet-switched bearers in, for example, the so-called 2.5 and third generation networks (2.5G and 3G) enabled network operators to provide data services as well as voice services. Eventually, network architectures will likely evolve toward all Internet Protocol (IP) networks which provide both voice and data services. However, network operators have a substantial investment in existing infrastructures and would, therefore, typically prefer to migrate gradually to all IP network architectures in order to allow them to extract sufficient value from their investment in existing infrastructures. Also to provide the capabilities needed to support next generation radio communication applications, while at the same time using legacy infrastructure, network operators could deploy hybrid networks wherein a next generation radio communication system is overlaid onto an existing circuit-switched or packet-switched network as a first step in the transition to an all IP-based network. Alternatively, a radio communication system can evolve from one generation to the next while still providing backward compatibility for legacy equipment.
One example of such an evolved network is based upon the Universal Mobile Telecommunication System (UMTS) which is an existing third generation (3G) radio communication system that is evolving into High Speed Packet Access (HSPA) technology. Yet another alternative is the introduction of a new air interface technology within the UMTS framework, e.g., the so-called Long Term Evolution (LTE) technology. Target performance goals for LTE systems include, for example, support for 200 active calls per 5 MHz cell and sub 5 ms latency for small IP packets. Each new generation, or partial generation, of mobile communication systems add complexity and abilities to mobile communication systems and this can be expected to continue with either enhancements to proposed systems or completely new systems in the future.
3GPP LTE is a fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP). The Universal Terrestrial Radio Access (UTRA) Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment (UE) 150, or any wireless device, is wirelessly connected to a Radio Base Station (RBS) 110a commonly referred to as a NodeB (NB) in UMTS, and as an evolved NodeB (eNodeB or eNB) in LTE, as illustrated in FIG. 1. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. In E-UTRAN, the eNodeBs 110a-c are directly connected to the core network (CN) 190. The eNodeBs 101a-c are also connected to each other via an X2 interface.
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 2a, where each resource element 210 corresponds to one OFDM subcarrier 220 during one OFDM symbol interval 230. In the time domain, LTE downlink transmissions are organized into radio frames 270 of 10 ms, each radio frame consisting of ten equally-sized subframes 250 of length Tsubframe=1 ms as shown in FIG. 2b. Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, also called Physical Resource Blocks (PRB), where a resource block corresponds to one slot 260 of 0.5 ms in the time domain and twelve contiguous subcarriers 220 in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. Downlink transmissions are dynamically scheduled, i.e., in each subframe the eNodeB transmits control information indicating to which terminals and on which resource blocks the data is transmitted during the current downlink subframe. This control signaling is typically transmitted in the first one, two, three or four OFDM symbols in each subframe.
The interest in deploying low-power nodes, such as pico base stations, home eNodeBs, relays, or remote radio heads, for enhancing the macro network performance in terms of the network coverage, capacity and service experience of individual users has been constantly increasing over the last few years. At the same time, there has been realized a need for enhanced interference management techniques to address the arising interference issues caused, for example, by a significant transmit power variation among different cells and cell association techniques developed earlier for more uniform networks.
In 3GPP, heterogeneous network deployments have been defined as deployments where low-power nodes of different transmit powers are placed throughout a macro-cell layout, implying also non-uniform traffic distribution. Such deployments are, for example, effective for capacity extension in certain areas, so-called traffic hotspots, i.e. small geographical areas with a higher user density and/or higher traffic intensity where installation of pico nodes can be considered to enhance performance. Heterogeneous deployments may also be viewed as a way of increasing the density of networks to adapt for the traffic needs and the environment. However, heterogeneous deployments also bring challenges for which the network has to be prepared to ensure efficient network operation and superior user experience. Some challenges are related to the increased interference in the attempt to increase small cells associated with low-power nodes, also known as cell range expansion.
Cell Range Expansion
The need for enhanced Inter-Cell Interference Coordination (ICIC) techniques is particularly crucial when the cell assignment rule diverges from the Reference Signal Received Power (RSRP)-based approach. This is e.g. the case when a path loss- or a path gain-based approach is used. This approach is sometimes also referred to as the cell range expansion, when it is adopted for cells with a transmit power lower than neighbour cells. The idea of the cell range expansion is illustrated in FIG. 3, where the cell range expansion of a pico cell served by a pico BS 110b is implemented by means of a delta-parameter Δ. The expanded cell range of the pico BS 110b corresponds to the outermost cell edge 120b, while the conventional RSRP-based cell range of pico BS 110b corresponds to the innermost cell edge 120a. The pico cell is expanded without increasing its power, just by changing the reselection threshold. In one example, the UE 150 chooses the cell of pico BS 110b as the serving cell when RSRPb+Δ≧RSRPa, where RSRPa is the signal strength measured for the cell of macro BS 110a and RSRPb is the signal strength measured for the cell of pico BS 110b. The striped line 130a illustrates RSRPa from the macro BS 110a, the dotted line 130b illustrates RSRPb from the pico BS 110b corresponding to the cell range 120a, and the solid line 130c illustrates the received signal strength from the pico BS 110b with the delta parameter added. This results in a change from the conventional cell range 120a to an expanded cell range 120b when Δ>0. Such cell range expansion is of interest in heterogeneous networks, since the coverage of e.g. pico cells may otherwise be too small and the radio resources of these nodes may be underutilized. However, as a result a UE may not always be connected to the strongest cell when it is in the neighborhood of a pico cell. The UE may thus receive a stronger signal from the interfering cell compared to the signal received from the serving cell. This results in a poor signal quality in downlink when the UE is receiving data at the same time as the interfering base station is transmitting.
Interference Management for Heterogeneous Deployments
To ensure reliable and high-bit rate transmissions, as well as robust control channel performance, good signal quality must be maintained in wireless networks. The signal quality is typically determined by the received signal strength and its relation to the total interference and noise received by the receiver. A good network plan, which among other factors also includes cell planning, is a prerequisite for the successful network operation. However, a network plan is static. For more efficient radio resource utilization, the network plan has to be complemented at least by semi-static and dynamic radio resource management mechanisms, which are also intended to facilitate interference management, and deployment of advanced antenna technologies and algorithms.
One way to handle interference is, for example, to adopt more advanced transceiver technologies, e.g. by implementing interference cancellation mechanisms in terminals. Another way, which can be complementary to the former, is to design efficient interference coordination algorithms and transmission schemes in the network.
Some ICIC methods for coordinating data transmissions between cells have been specified in LTE release 8, where the exchange of ICIC information between cells in LTE is carried out via the X2 interface by means of the X2-AP protocol. Based on this information, the network can dynamically coordinate data transmissions in different cells in the time-frequency domain and also by means of power control so that the negative impact of inter-cell interference is minimized. With such coordination, base stations may optimize their resource allocation by cells either autonomously or via another network node ensuring centralized or semi-centralized resource coordination in the network. With the current 3GPP specification, such coordination is typically transparent to wireless devices. Two examples of coordinating interference on data channels are illustrated in FIGS. 4a-b. The figures illustrate a frame structure for three subframes, carrying the periodically occurring Cell specific Reference Signals (CRS) 420, and with a control channel region 410 in the beginning of each subframe, followed by a data channel region 430. The control and data channel regions are white when not carrying any data and filled with a structure otherwise. In the first example illustrated in FIG. 4a, data transmissions in two cells belonging to different layers are separated in frequency. The two layers may e.g. be a macro and a pico layer respectively. In the second example illustrated in FIG. 4b, low-interference conditions are created at some time instances for data transmissions in pico cells. This is done by suppressing macro-cell transmissions in these time instances, i.e. in so called low-interference subframes 440, in order to enhance performance of UEs which would otherwise experience strong interference from macro cells. One example is when UEs are connected to a pico cell but are still located close to macro cells. Such coordination mechanisms are possible by means of coordinated scheduling, which allows for dynamic interference coordination. There is e.g. no need to statically reserve a part of the bandwidth for highly interfering transmissions.
In contrast to user data, ICIC possibilities for control channels and reference signals are more limited. The mechanisms illustrated in FIGS. 4a-b are e.g. not beneficial for control channels. Three known approaches of enhanced ICIC (e-ICIC) to handle the interference on control channels are illustrated in FIGS. 5a-c. The approaches illustrated in FIGS. 5a and 5c require standardization changes while the approach illustrated in FIG. 5b is possible with the current standard although it has some limitations for Time Division Duplex (TDD) systems, is not possible with synchronous network deployments, and is not efficient at high traffic loads. In FIG. 5a, low-interference subframes 540 are used in which the control channels 550 are transmitted with reduced power for the channels. In FIG. 5b, time shifts are used between the cells, and in FIG. 5c in-band control channels 560 are used in combination with a control of the frequency reuse.
The basic idea behind interference coordination techniques as illustrated in FIGS. 4a-b and FIGS. 5a-c is that the interference from a strong interferer, such as a macro cell, is suppressed during another cell's—e.g. a pico cell's—transmissions. It is assumed that the pico cell is aware of the time-frequency resources with low-interference conditions and thus can prioritize scheduling in those subframes of the transmissions for users which are likely to suffer most from the interference caused by the strong interferers. The possibility of configuring low-interference subframes, also known as Almost Blank subframes (ABS), in radio nodes and exchanging this information among nodes, as well as time-domain restricted measurement patterns restricting UE measurements to a certain subset of subframes signaled to the UE, have recently been introduced in the 3GPP standard (TS 36.423 v10.1.0, section 9.2.54, and 3GPP TS 36.331 v10.1.0, section 6.3.6, respectively). An eNodeB may thus transmit ABS which are subframes with reduced power and/or reduced activity on some physical channels, in order to allow the UE to perform measurements under low-interference conditions.
With the approaches illustrated in FIGS. 4a-b and FIGS. 5a-c, there may still be a significant residual interference on certain time-frequency resources, e.g., from signals whose transmissions cannot be suppressed, such as CRS or synchronization signals. Some known techniques to reduce interference are:                Signal cancellation, by which the channel is measured and used to restore the signal from a limited number of the strongest interferers. This has impacts on the receiver implementation and its complexity. In practice, channel estimation puts a limit on how much of the signal energy that can be subtracted.        Symbol-level time shifting. This technique has no impact on the standard, but is not relevant e.g. for TDD networks and networks providing the Multimedia Broadcast Multicast Service (MBMS) service. This is also only a partial solution to the problem since it allows to distribute interference and avoid it on certain time-frequency resources, but not to eliminate it.        Complete signal muting in a subframe. It could e.g. be not to transmit CRS and possibly also other signals in some subframes. This technique is non-backward compatible to Rel. 8/9 UEs which expect CRS to be transmitted, at least on antenna port 0 in every subframe, even though it is not mandated that the UE performs measurements on those signals every subframe.        
Using MBSFN subframes with no MBMS transmissions, which will hereinafter be referred to as blank MBSFN subframes, is a backwards compatible approach that achieves the effect similar to that with complete signal muting, since no signals, not even CRS, are transmitted in the data region of a blank MBSFN subframe. Although CRS are still transmitted in the first symbol of the first slot of a blank MBSFN, using blank MBSFN subframes to avoid potential interference from strongly interfering cells may still be an attractive approach for at least some network deployments.
Restricted Measurement Pattern Configuration Used for Enhanced Inter-Cell Interference Coordination (eICIC)
To facilitate measurements in an expanded cell range, i.e., where high interference is expected, the standard specifies ABS patterns for eNodeBs, as described above, as well as restricted measurement patterns for UEs. An ABS pattern is a transmission pattern at the radio base station which is cell-specific. The ABS pattern may be different from the restricted measurement patterns signaled to the UE.
To enable restricted measurements for Radio Resource Management (RRM), Radio Link Management (RLM), Channel State Information (CSI), as well as for demodulation, the UE may receive the following set of patterns via Radio Resource Control (RRC) UE-specific signaling. The set of patterns are described in TS 36.331 v10.1.0, sections 6.3.2, 6.3.5, and 6.3.6:                Pattern 1: A single RRM/RLM measurement resource restriction pattern for the serving cell.        Pattern 2: One RRM measurement resource restriction pattern per frequency for neighbour cells (up to 32 cells). The RRM measurement is currently only defined for the serving frequency.        Pattern 3: A resource restriction pattern for CSI measurement of the serving cell with two subframe subsets configured per UE.        
The pattern is a bit string indicating restricted subframes, where the pattern is defined by a length and a periodicity. The restricted subframes are the subframes indicated by a measurement resource restriction pattern in which the UE is allowed or recommended to perform measurements. The length and periodicity of the patterns are different for Frequency Division Duplex (FDD) and TDD (40 subframes for FDD and 20, 60 or 70 subframes for TDD).
Restricted measurement subframes are configured to allow the UE to perform measurements in subframes with improved interference conditions. Improved interference conditions may e.g. be implemented by configuring ABS patterns at interfering radio nodes such as macro eNodeBs. A pattern indicating such subframes with improved interference conditions may then be signaled to the UE in order for the UE to know when it may measure a signal under improved interference conditions. The pattern may be interchangeably called a restricted measurement pattern, a measurement resource restriction pattern, or a time domain measurement resource restriction pattern. As explained above, an ABS is a subframe with reduced transmit power or activity. In one example, an MBSFN subframe may be an ABS, although it does not have to be an ABS and the MBSFN subframe may even be used for purposes other than interference coordination in the heterogeneous network. ABS patterns may be exchanged between eNodeBs, e.g., via X2, but these eNodeB transmit patterns are not signaled to the UE. However, an MBSFN configuration is signaled to the UE. Signaling independent of the eICIC patterns is used for configuring MBSFN subframes in the UE, via System Information Block (SIB) Type 2 (SIB2).
In a general case, Physical Downlink Shared Channel (PDSCH) transmissions are allowed in ABS subframes, but it is left up to the network implementation how interference is coordinated across the network in these subframes. UEs in Rel-8/9 transmission mode cannot receive PDSCH in MBSFN subframes. This may be exploited e.g. for energy saving. Rel-10 UEs will support PDSCH transmissions in MBSFN subframes, but only UEs in specific transmission modes—transmission mode 9 (TM 9)—will be able to receive DownLink (DL) assignments in signaled MBSFN subframes. These UEs will have to monitor Physical Downlink Control Channel (PDCCH) to check whether there is a DL assignment on a DL Shared Channel (SCH) for this UE. These UEs are also capable of receiving demodulation reference signals for channel estimation, and the need for CRS can thus be avoided.
Random Access
Another aspect of interest for this discussion involves Random Access Channel (RACH) transmissions in E-UTRAN. The Random Access (RA) procedure in LTE is performed to enable the UE to gain uplink access under the following scenarios (see e.g. 3GPP TS 36.300 V10.3.0 (2011-03) section 10.1.5):                During an initial access in idle mode;        For RRC connection re-establishment, e.g. after a radio link failure, or a handover failure;        After the UE has lost uplink synchronization;        Due to data arrival when UE in connected mode does not retain UpLink (UL) synchronization e.g. due to long Discontinuous Reception (DRX);        During HandOver (HO);        
RA may also be used to facilitate positioning measurements, e.g. for performing eNodeB Rx-Tx time difference measurement which in turn is used for deriving a timing advance value.
There are various types of RA procedures. The RA procedure can be either contention based or non-contention based. The contention based RA is used during initial access, for RRC connection re-establishment, to regain uplink synchronization and for data transmission when there is no uplink synchronization. On the other hand the non-contention based RA is used during HO and for positioning measurements. Both contention and non-contention RA mechanisms comprise of multi-step procedures.
In contention based RA the UE randomly selects the RA preamble during the RACH opportunity to the eNodeB. During the second step the network responds to the UE with at least a RA preamble identifier, and an initial uplink grant in the RA Response (RAR) message. During the third step the UE uses the initial uplink grant or allocation received in RAR to transmit further details related to the connection request in a message also known as a message 3 (msg3). In message 3 the UE also sends its identifier, which is echoed by the eNodeB in the contention resolution message during the fourth and final step. The contention resolution is considered successful if the UE detects its own identity in the contention resolution message.
In non-contention based RA the eNodeB first assigns a RA preamble. During the second step the UE sends the assigned preamble during the RACH opportunity to the eNodeB. During the third step the network responds to the UE with at least a RA preamble identifier, and an initial uplink grant in the RAR message. The UE uses the initial allocation received in RAR to transmit further details related to for example HO. In case of non-contention based RA there is no contention resolution phase.
Measurements in Radio Communication Systems
Yet another aspect of interest for this discussion involves measurements which are performed in radio communication systems. In LTE the measurements are done for various purposes, such as for mobility, also known as RRM measurements, for positioning, for Self-Organizing Networks (SON), and for Minimization of Drive Tests (MDT). The well-known intra-LTE mobility measurements, which may be both intra-, and inter-frequency, are: RSRP and Reference Signal Received Quality (RSRQ). The well-known inter-RAT mobility measurements are:                UTRAN Common Pilot Channel (CPICH) Received Signal Code Power (RSCP)        UTRA carrier Received Signal Strength Indicator (RSSI)        UTRAN CPICH Ec/No, where CPICH Ec/No=CPICH RSCP/carrier RSSI        GSM carrier RSSI        CDMA2000 Pilot Strength        High Rate Packet Data (HRPD) Pilot Strength.        
In terms of positioning measurements, the following positioning measurements are possible since Rel-9 with enhanced cell ID and Observed Time Difference Of Arrival (OTDOA) positioning methods:                UE Rx-Tx time difference measurement        eNodeB Rx-Tx time difference measurement        Timing advance (TA) measurement        Angle of Arrival (AoA) measurements        Reference signal time difference (RSTD) for OTDOA        RSRP and RSRQ.        
The MDT feature has been introduced in LTE and HSPA Rel-10. The MDT feature provides means for reducing the effort for operators when gathering information for the purpose of network planning and optimization. The MDT feature requires that the UEs log or obtain various types of measurements, events and coverage related information. The logged or collected measurements or relevant information are then sent to the network. According to the traditional approach, the operator has to collect similar information by means of the so called drive tests and manual logging. The MDT feature is described in 3GPP TS 37.820. The UE can collect the measurements during connected state as well as in low activity states such as idle state in UTRA/E-UTRA, cell PCH states in UTRA. A few examples of potential MDT UE measurements are:                Mobility measurements e.g. RSRP, RSRQ;        RA failure;        Paging Channel Failure (PCCH Decode Error);        Broadcast Channel failure;        Radio link failure report.        
The E-UTRAN also employs the concept of a SON. The objective of the SON entity is to allow operators to automatically plan and tune the network parameters and configure the network nodes. The conventional method is based on manual tuning, which consumes enormous amount of time, and resources and requires considerable involvement of work force. In particular due to network complexity, to a large number of system parameters, and to IRAT technologies, it is very attractive to have reliable schemes and mechanism which could automatically configure the network whenever necessary. This can be realized by SON, which can be visualized as a set of algorithms and protocols performing the task of automatic network tuning, planning, configuration, parameter settings. In order to accomplish this, the SON node requires measurement reports and results from other nodes, such as the UE, or the RBS.
In general the heterogeneous network under consideration is distinguished by the time sharing of the radio resources between a high power network node, also known as a macro node such as a macro eNodeB, and a low-power network node, such as a pico eNodeB, a micro eNodeB, or a Home eNodeB, as described above. The time sharing of resources between the high- and low-power nodes is done in the downlink and/or uplink. It would be desirable to provide techniques, mechanisms, methods, devices, software and systems which, for example, exploit heterogeneous network deployment and configuration scenarios to enhance the performance.